1. Field of the Invention
The present invention relates to a method for producing a substrate having a minute or fine pattern by use of a transfer roll, a method for producing an organic EL element (Organic Electro-Luminescence element or organic light emitting diode) using the same, and the organic EL element obtained thereby. More specifically, the present invention relates to a method for producing a substrate having a minute or fine pattern by use of a transfer roll by a sol-gel method, a sol used for the producing method, a method for producing an organic EL element using the producing method, and the organic EL element obtained thereby.
2. Description of the Related Art
There has been known a lithography method as a method for forming a minute pattern such as a semiconductor integrated circuit. The resolution of the pattern formed by the lithography method is dependent on the wavelength of a light source and the numerical aperture of an optical system, and the light source is expected to have shorter wavelength in order to meet demand for miniaturized devices in recent years. However, the light source having the short wavelength is expensive, development thereof is not easy, and the development of an optical material transmitting such a short-wavelength light is also needed. Further, manufacturing a large-area pattern by a conventional lithography method needs a large-size optical element, and thus there are difficulties in technical and economic aspects. Therefore, a novel method for forming a desired pattern on a large area has been studied.
There has been known a nanoimprint method as a method for forming a minute pattern without using any conventional lithography apparatus. The nanoimprint method is a technique such that a pattern of an order of nanometer can be transferred by sandwiching a resin between a mold and a substrate. A thermal nanoimprint method, a photonanoimprint method, and the like have been studied depending on an employed material. Of the above methods, the photonanoimprint method includes four steps of: i) application of a resin layer; ii) pressing by use of the mold; iii) photo-curing; and iv) mold-releasing. The photonanoimprint method is excellent in that processing on a nanoscale can be achieved by the simple process as described above. Especially, since a photo-curable resin curable by being irradiated with light is used as the resin layer, a period of time for a pattern transfer step is short and a high throughput is promised. Thus, the photonanoimprint method is expected to come into practical use in many fields including, for example, an optical member such as the organic EL element and LED, MEMS, a biochip and the like, in addition to a semiconductor device.
However, the photo-curable resin described above generally has a low heat resistance, and decomposed and/or turns into yellow at a high temperature. Thus, there is fear that a film having the minute pattern is decomposed in a case that a high temperature treatment is included in subsequent steps. Further, the photo-curable resin has a low adhesion property to a glass substrate. Furthermore, in a case that the resin layer to which the pattern has been transferred is used for an element such as the organic EL element, there is fear that impurities are eluted from the resin layer and cause adverse effect on the element.
As the thermal nanoimprint method, for example, there has been known the method as described in Japanese Patent Application Laid-open No. 2001-26052. That is, it is prepared a mold having a planar transfer surface on which a concavity and convexity pattern is formed; a thermoplastic base member (processed object) is heated and softened; the transfer surface is pressured and pressed against the base member; the mold and the base member are cooled as they are; the mold is peeled off from the base member; and thereby an inverted pattern is transferred. The thermal nanoimprint method has advantages such that the nano-level transfer can be achieved by the simple method as described above, and that there is a wide range of choice of the transfer-target base member.
However, the thermal nanoimprint method generally needs a high pressure for pressing, and takes time for a heating-cooling cycle. Thus, the thermal nanoimprint method is unsuitable for a case in which high productivity is desired. Further, the thermal nanoimprint method has the problem of the heat resistance since the thermoplastic resin is heated and softened to perform the transfer. For example, in a case that the transfer-target object is exposed to a temperature higher than a molding temperature, there is a fear that the pattern is deformed and thereby it can not be used.
A method for improving the heat resistance is exemplified by a nanoimprint method using a thermosetting material. For example, there has been known the method as described in Japanese Patent Application Laid-open No. 2008-049544. That is, a resist film is applied on a substrate, the substrate is pressed with a mold having a flat plate shape, and then the resist film is cured using a heater. Especially, a nanoimprint molded product using an inorganic sol-gel material has a high heat resistance, and any problem is unlikely to occur even when a high temperature treatment is performed. However, the pressing method using the sol-gel material also has the following problem. Each thermal expansion coefficient of the flat-plate-shaped mold, the base member, and a stage is different from one another, thus the difference in the linear coefficients of expansion causes deviation in a transferred surface profile of the molded product. In order to suppress the deviation, a means for absorbing the thermal expansion and/or a long period of heating-cooling process is/are required. Further, since a solvent positioned in the center of the mold having the flat plate shape used for the pressing is less likely to evaporate, transfer failure occurs in the center of the mold and/or unevenness of the transfer occurs owing to the difference in a cured state of the surface. Furthermore, in a case that gas is generated due to bumping of the solvent, bubbles are formed in the pattern and/or a trace or mark of gas (a trace of escape of gas) is left in some cases. The above problems become especially conspicuous when the transfer pattern is formed in a large area. The solvent evaporates from an end portion of the coating film, and a heating process is required for a long time at a low temperature (for example, 120 degrees Celsius) in order to evaporate the solvent completely, which decreases the productivity. Further, since the surface of the mold having the flat plate shape needs to be pressed uniformly at the same time, a relatively great transfer pressure is required. Thus, when the transfer pattern has the large area, it is difficult to press the surface of the mold uniformly under the great pressure. Moreover, when the mold is peeled off from the base member, a great peeling force is required to pull up the mold in a vertical direction. Therefore, there is fear that a sol-gel material layer is broken at the time of the peeling of the mold from the sol-gel material layer, and thereby the pattern is collapsed. As described above, there are many problems to perform the press transfer to a large area by use of the mold having the flat plate shape.
Instead of the pressing method using the flat-plat-shaped mold, there has been known the roll press method, as described in Japanese Patent Application Laid-open No. 2010-269480, in which a pressing roll and a cylindrical master plate for duplication having a minute concavity and convexity pattern are used. An area of contact between a mold and a coating film is small in a roll process as compared with the case in which the flat-plate-shaped mold is used, and thus it is considered that some of the above problems can be solved. However, the roll press method using the sol-gel material also has the following problem. In processing using the sol-gel material, the processing is started from a raw material solution; after production of sol, gel is produced by chemical reaction such as hydrolysis and condensation polymerization; the solvent remained inside is removed by a heating process; and the glass and ceramics is obtained by further promoting densification. However, since the time-dependent change of evaporation of the solvent in a gelled state is rapid, if the control is not performed precisely after the sol is applied on the substrate and before the concavity and convexity pattern is pressed by the roll press, the failure arises such that depths of concavities and convexities are insufficient and that the transfer pattern can not be obtained.
In view of the above, an object of the present invention is to provide a method for reliably producing a substrate having a minute or fine concavity and convexity pattern by use of a sol-gel material by a roll press method with high efficiency. Another object of the present invention is to provide an organic EL element having excellent durability and satisfactory light resistance. Still another object of the present invention is to provide a sol suitable for the method for producing the substrate of the present invention.